This invention relates to a method of assigning a radio channel to a calling a mobile body of mobile communication system and a radio channel assigning system for the mobile communication system, and more particularly to a radio channel assigning method and system wherein a service area is divided into a plurality of radio zones which overlap with each other and a mobile-system base station is located in each radio zone and wherein a control channel and speech channels are assigned to each mobile-system base station (MBS) so as to effect common control thereof.
In recent years the requirement for communicating with such mobile bodies as motor cars, ships or portable telephone set, etc. by using ordinary telephone networks has been increasing, and many types of mobile communications systems have been developed to fulfil such requirement.
One example of such mobile communication systems is shown in FIG. 1 in which a service area is divided into a plurality of radio zones and a mobile-system base station equipped to transmit a particular electric wave frequency assigned thereto is located in each zone. The reason for using a control channel is that, where the system has a large capacity and a mobile communication service over a wide area is contemplated, the control efficiency is increased by commonly controlling a plurality of speech channels at a high speed thereby increasing the efficiency of utilizing the speech channels. Where the service area is divided into a plurality of small service areas and where mobile-system base stations are located in respective service areas, the base stations are not required to have large transmission power whereby it is possible to construct them as compact units. This arrangement permits repeated use of a frequency in different zones thereby enabling efficient use of a limited number of frequencies to increase the processing quantity of the traffic as compared to a system wherein the service area is not so divided.
In the example of the prior art radio channel assigning system for a mobile communication system shown in FIG. 1 the service area is divided into n small zones and mobile-system base stations 10.sub.1 through 10.sub.n are located in respective zones. The electric wave transmitted from each station serves each zone. As can be noted from FIG. 1 adjacent zones overlap each other and to the ith mobile base station 10.sub.i (where i = 1, . . . n) are assigned a control channel F.sub.i (i = 1, . . . n) and speech channels f.sub.il, . . . f.sub.im where i = 1, . . . n and m is an integer. In a vehicle, these channels are switched. In this manner, each mobile-system base station is normally transmitting control signals through a particular control channel assigned thereto.
Where a vehicle 12 in one service area of the system requests communication, the vehicle catches or selects a mobile-system base station from which it receives a signal wave having the maximum field strength or intensity among control channels corresponding to respective mobile-system base stations, for example station 10.sub.1, whereby the vehicle transmits a call or originating signal through a channel F.sub.1 assigned to the mobile-system base station 12. Such channel selection or assignment is performed manually or automatically. The call signal transmitted by the vehicle 12 is received and relayed by only the mobile-system base station 10.sub.1 and transmitted to the mobile control unit 20 which operates according to the flow chart shown in FIG. 2. More particularly, upon receiving the call signal the processing advances from a start step 21 to a waiting step 22. Under these conditions, the mobile-system control unit 20 stores the idle and busy states of the speech channels of all mobile-system base stations 10.sub.1 through 10.sub.n. When the call or originating signal from the vehicle is received at this time (step 23 shown in FIG. 2), the mobile-system control unit 20 determines from which mobile base station the signal has originated based on the particular frequencies of the used control channels as shown by step 24 shown in FIG. 2. Then the processing is advanced to step 25 shown in FIG. 2 for checking whether there is an idle channel in the selected mobile base station or not. When there is an idle channel it is designated (step 26, shown in FIG. 2) and the information identifying the designated idle channel is transmitted through the mobile base station 10.sub.1 via the control channel F.sub.1. The vehicle 12 receives the information regarding the idle speech channel and the designated idle speech channel is manually or automatically selected for communication, which includes the dial operation of an ordinary telephone set.
When the mobile control unit 20 selects an idle speech channel of the corresponding mobile base station 10.sub.1 upon receiving the call signal from the vehicle 12 through station 10.sub.1 and control channel F.sub.1, there on occasion will be a case wherein the mobile base station is busy so that there is no available idle speech channel. At this time, processing proceeds from step 25 to step 27 to perform a call failure processing so as to connect the vehicle 12 to the mobile base station 10.sub.1 via the mobile control unit 20 thereby transmitting a signal indicating that the mobile base station is busy and no idle speech channel is available.
With this system, however, the probability of assigning the idle speech channel for a calling vehicle is determined by the capacity of the speech channel assigned to a mobile-system base station which receives a signal from a calling vehicle at the maximum field intensity. In order to increase the probability it is necessary to increase the number of speech channels assigned to each mobile-system base station. This requires a large expense of installation and presents the following problems. For example, in the service area shown in FIG. 1 there will be a case wherein a plurality of vehicles concentrate in the radio zone of the mobile-system station 10.sub.1 and consequently there is a small number of vehicles in the other zones. In such a case even when there is an idle speech channel in a mobile-system base station adjacent to the mobile-system base station 10.sub.1, as the control channels assigned to respective mobile-system base stations are different (for example, channel F.sub.2 to station 10.sub.2 and channel F.sub.3 to station 10.sub.3) even though there is an idle speech channel in the adjacent mobile-system base station the mobile-system control unit 20 can not designate the idle speech channel. In such a case the utilization efficiency of the channel is low.